The present invention relates to valves in general, especially to ball, plug or slide valves, and more particularly to improvements in valves which are intended to control the flow of hot and extremely hot fluids, e.g., in nuclear reactor plants and similar institutions. Still more particularly, the invention relates to improvements in annular sealing members (hereinafter called seats for short) which are used in such valves to sealingly engage the valving element at least while the latter assumes its closed position.
Valves which are used to control the flow of hot fluids are subjected to pronounced stresses, especially in the region where the valving element engages the seat. If the temperature of fluid and/or the surrounding atmosphere is in the range of several hundred .degree.C. (e.g., between 500 and 600.degree. C.), the valving element as well as the seat must be made of a metallic material. At such elevated temperatures, the valving element cannot be allowed to slide with respect to the seat because any sliding movement would result in substantial damage to or in immediate destruction of complementary sealing surfaces on the valving element and the seat. If the temperature exceeds a certain upper limit, the complementary surfaces are likely to become welded to each other.
It is already known to move the normally ring-shaped seat away from the valving element before the latter is caused to change its position by performing a turning, sliding and/or other movement. The seat is returned into sealing engagement with the valving element when the latter reaches and dwells in the newly selected position. Such measures are necessary in nuclear reactor plants and elsewhere when the temperature to which the seat and/or the valving element is exposed is sufficiently high to risk damage to or destruction of complementary sealing surfaces during movement of the valving element while the latter contacts the seat. The seat can be moved away from the valving element by resorting to hydraulical, pneumatic and/or mechanical moving means. In most instances, the seat is connected with a bellows whose interior can be filled with a pressurized fluid to urge the sealing surface of the seat against the sealing surface of the valving element. As a rule, the seat is moved axially in the interior of the valve body. It is also known to mount the seat on a ring-shaped piston which is installed in a cylindrical chamber adapted to recieve pressurized fluid from a source located externally of the valve body.
Additional problems arise when the valve is designed for use in regions where the temperature fluctuates within an extremely wide range, i.e., where the temperature is likely to rise to several hundred .degree.C. or drop to a fraction of such value. Pronounced fluctuations of the temperature of the surrounding atmosphere are likely to result in radial shifting of the annular seat, i.e., a seat which is properly centered at a relatively low temperature is likely to be shifted radially in response to a pronounced rise in temperature, especially if the temperature is not uniform in the entire region around the seat. Radial or lateral shifting or tilting of the seat is likely to affect the sealing action when the seat is moved into engagement with the valving element. The likelihood of improper sealing action is further enhanced due to thermally induced deformation of the seat and/or due to unequal thermal stressing of such part. In many instances, minor or even minute movements of the seat from its optimum position and/or minor or minute distortion of the seat might result in pronounced leakage of contaminated fluid when the valve is used in a nuclear reactor plant. In other words, proper centering of the seat is of utmost importance, and such proper centering must be insured irrespective of the fact that the valve is likely to be used to regulate the flow of extremely hot fluids and/or that the temperature around the component parts of the valve is likely to fluctuate within an extremely wide range. The aforediscussed conventional means for moving the seat into and from sealing engagement with the valving element cannot insure proper centering of the seat under the just outlined circumstances. If the seat is mounted on an annular piston, the accuracy of centering depends to a large extent on the quality of bearings for the piston, on the extent of deformation of the valve body and/or on the wear upon the surfaces along which the piston slides while moving the seat to or from the valving element. The aforementioned tilting, thermally induced deformation or uneven stressing of the seat is likely to permit leakage of excessive quantities of fluid and/or direct contact between the metallic materials of the seat and valving element while the latter moves between open and closed positions.
The proposal to simply suspend the seat on a bellows also fails to eliminate the likelihood of improper centering of the seat because the bellows, too, is likely to undergo thermally induced deformation. Moreover, the pressure of fluid which is to cause the bellows to move the seat is likely to fluctuate and the weight of the bellow (and/or of the seat which is suspended thereon) is likely to affect the centering action. Therefore, the aforedescribed conventional valves cannot be used in nuclear reactor plants or for other applications where a satisfactory sealing action must be guaranteed at extremely high temperatures and/or under circumstances when the temperatures fluctuate within an extremely wide range.